1. Field of the Invention
This invention in one embodiment relates to the chemical treatment of cellulose fibers to impart the fiber with improved dispersibility and reinforcing efficiency in fiber reinforced composite materials. More particularly, in one embodiment, this invention relates to cellulose fiber reinforced cement composite materials using chemically treated fibers with improved dispersibility, including fiber treatment methods, formulations, methods of manufacture and final products with improved material properties relating to the same.
2. Description of the Related Art
Fiber-reinforced cement products such as building sheets, panels, planks and roofing have been used for building construction for more than one hundred years. Reinforcing fibers used in such building products include asbestos fibers, cellulose fibers (see, e.g., Australian Patent No. 515151, U.S. Pat. No. 6,030,447), metal fibers, glass fibers and other natural or synthetic fibers. In recent years, the use of asbestos fibers has decreased substantially due to health concerns associated with the exposure and inhalation of asbestos fibers. As a viable alternative, wood cellulose has become one of the predominant fibers used in commercial fiber-reinforced building materials because it is an effective, low cost, renewable natural reinforcement fiber compatible with common fiber cement manufacturing processes, including the autoclave process.
However, cellulose reinforced fiber cement materials can have performance drawbacks such as lower reinforcing efficiency, lower strength and toughness due to poor fiber dispersion and uneven fiber distribution in the cement mix. These drawbacks are largely due to the hydrophilic nature of cellulose fibers. It is generally understood that cellulose fibers are primarily polysaccharides comprised of five or six carbon sugars that have multiple hydroxyl and carboxyl functional groups. These functional groups provide cellulose fibers with a strong tendency to form hydrogen intra-fiber and inter-fiber bonds. Hydrogen bonding between fibers often results in the formation of fiber clumps or clusters. The fiber clusters are difficult to disperse in a cementitious mixture even with the help of hydrapulping and refining processes as described in Australian Patent No. 515151. These fiber clusters are even more difficult to disperse in dry and semi-dry processes such as extrusion, molding, Magnani and casting. Moreover, hydrogen bonding between different hydroxyl groups of the same fiber is likely to promote fiber curling or forming fiber balls, which can also result in lower fiber reinforcement efficiency.
For example, when the fibers are dried in the process of forming sheets, the hydrogen bonding within and among cellulose molecules is sufficiently strong such that complete dispersion or fiberization of the dried fibers by mechanical means is extremely difficult to achieve. Use of poorly dispersed or fiberized fibers in fiber cement composite materials usually results in uneven fiber distribution and lower reinforcing efficiency, which in turn can lead to lower strength, toughness, and strain in the final fiber cement product. Thus, in order to achieve a certain level of reinforcement, substantially more fibers are needed to compensate for the uneven fiber distribution in the cementitious matrix, which in turn can significantly increase the material cost.
A number of prior art references disclose methods of improving fiber dispersion in a cementitious mix. However, all of these references are directed toward using mechanical action to break the bonds between fibers. For example, U.S. Pat. No. 3,753,749 to Nutt discloses milling or otherwise mechanically preparing the fibers beforehand so that the fibers can be uniformly distributed in a concrete mix. U.S. Pat. No. 5,989,335 to Soroushian discloses using mechanical action to reduce the bonding between fibers so that the fibers can be dispersed in conventional concrete mixes. One disadvantage of using mechanical means to break the inter-fiber bonding is that once the mechanically dispersed fibers are placed in the concrete mix, hydrogen bonds can again form between the fibers and cause the fibers to re-cluster in the mix.
In the paper industry, some research has been directed toward chemically treating cellulose fibers to reduce the fiberization energy needed to fiberize the pulp. Since high energy is typically required to fiberize pulp with strong inter-fiber hydrogen bonding, efforts have been made to reduce the hydrogen bonding among fibers in the pulp by adding organic and/or inorganic chemicals called debonders to lower the fiberization energy requirement. The debonders are typically surfactants but can also be inorganic fillers. These treated fibers have been developed primarily for applications in diaper and sanitary napkin manufacturing.
Thus far, these chemically treated fibers have been used exclusively in the paper industry for the purpose of reducing fiberization energy during fiberization processes such as hammermilling. There has been no motivation to use these chemically treated fibers to improve fiber dispersion as fiber dispersion is generally not a concern for the papermaking industry since the majority of the papermaking processes such as Fourdrinier, cylinder (Hatschek) and twin-wire use very dilute fiber slurry. The fiber consistencies in these slurries are typically between about 0.01% to 4%. At such low consistencies, water will break most of the inter-fiber hydrogen bonds while the remaining fiber clusters can be easily dispersed using mechanical means such as hydrapulping, pumping, deflakering and refining.
Poor fiber dispersion continues to pose a serious problem in the manufacture of fiber reinforced cement composite materials, especially when long fibers are used in a dry or semi-dry process wherein fiber dispersion is even more difficult to achieve. The fiber cement mixture typically has a solid content of about 30% to 80% by weight in a dry or semi-dry process such as extrusion, casting or molding processes. At such high solid concentrations, fiber dispersion cannot be achieved by dilution, solvency, or agitation. As a consequence, poorly dispersed fiber bundles or clusters often lead to severe defects in the final product, including a significant loss in mechanical properties. The high alkalinity of the aqueous fiber cement system (pH commonly higher than 10, also promotes the hydrogen bonding among fibers, which can make the fibers more difficult to disperse in a cementitious mixture than in most conventional paper-making systems where the pulp slurry is typically under acidic or neutral conditions.
Accordingly, there is a need for a fiber that can be readily dispersed and uniformly distributed in fiber reinforced composite building materials. There is also a need for a fiber reinforced building material having improved fiber distribution and reinforcing efficiency, and material formulations and processes for manufacturing the same.